Robert Snyder and Stephan Custer
Geographic Information and Analysis Center and Earth Sciences, Montana
State University, Bozeman, MT
Last Modified 7 June 2000; 02 January 2009
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Example Questions the Database Can Help Answer
1. What is the mean average precipitation expected for the area
2. What other climatic information is available for the area
3. How much recharge can be expected in the area under review?
(Note the amount of recharge
requires further climatic and soil analysis but requires data on
Map 1. Precipitation
- Mean annual precipitation contoured
for the period 1961-1990 for part of
- Mean annual precipitation
from contours for the period 1961-1990
for part of Gallatin County.
- Prism precipitation data for all of Montana
on the web (gif; jpg)
- Prism precipitation data for other states
on the web.
- Natural Resources Conservation Snow
Survey Unit Data for the mountains on the web
- Climate data by region in
on the web.
- NOAA Climate
- Bureau of Reclamation Agrimet
Mean Annual Precipitation is the total annual precipitation calculated
from all yearly total precipitation data for the period of interest
by the number of years in the period of interest. The values
depending on the period chosen. The data is usually reported for
30 year periods. The most recent period available is
The averages are updated every 10 to 20 years.
Mean Monthly Precipitation is the total monthly precipitation
calculated from all data monthly total precipitation data for the
of interest divided by the number of years in the period of interest.
Contoured mean annual precipitation was drawn by Phil Farnes based
all available precipitation data for the period 1961-1990. The
interval is 2 inches below the 20 inch contour and 10 inches above the
20 inch contour.
Interpolated mean annual precipitation is the precipitation
for each grid point based on the known values on the contour
The value at each individual grid point was calculated from the
mean annual precipitation using the
without drainage enforcement. No digital elevation model was used
to enforce the
contouring. Only the precipitation contours were used.
Snow Survey Unit Data. There are two organizations which
collect and summarize data. The National Oceanic and Atmospheric
Administration (NOAA) sites tend to be easily accessible and near where
people live. The Natural Resources Conservation (NRCS) Data tends
to be collected at remote high elevation sites. The NRCS data is
used for water supply forecasts in the Western US which are dependent
knowledge of mountain snow pack and rainfall information.
Precipitation includes both snow and rainfall expressed as
Precipitation in the winter is different than snow depth. Because
snow density changes from storm to storm and through the snow-fall
the snow depth does not reflect directly how much water is in the
Precipitation reflects the water in the snow and in rainfall.
About the Data
There are two precipitation maps. The contour map classifies
both as contour lines (line coverage) and as the mid-point value
the two contours. The interpolated map shows unique values for
grid point which were calculated from the contour-line data. The
contour map shown is the result of digitization of hand drawn contour
prepared by Phil Farnes for the period 1961-1990 using all available
data from all known precipitation stations (NOAA and NRCS) in the
The work is described in Custer, S.G., Farnes, P., J.P., Wilson, and
R.D., 1996, A comparison of hand- and spline-drawn precipitation maps
mountainous Montana. Water Resources Bulletin, v. 32(2), p.
The hand-drawn maps are presented because they were drawn by the same
that produced earlier official NRCS precipitation maps. The map
drawn on the Bozeman 1 x 2 degree sheet. The lines were
and the value of the mid point between contour lines was assigned to
grid points between contours. For example, if the contour lines
18 and 20 inches, the mid point value is 19 inches. The contour
on the map are not uniform. Below 20 inches, the map was drawn
two inch contours. Above 20 inches the map was drawn with ten
contours. The maps were hand drawn on a 1:250,000 Army Map
Service 1x2 degree topographic map (UTM, NAD27), digitized, and
into geographic co-ordinates. The areas between contours were
to mid-point values and projected again into UTM NAD83 co-ordinates.
map does not cover all of Gallatin County. The 1x2 degree sheet
not extend above 46 degrees north latitude nor below 45 degrees
latitude. No detailed digital data is available north of 46
and south of 45 degrees north latitude. The precipitation data
clipped so that the data falls between the north-south lines that bound
the eastern and western most extent of the county. Because
the contours provide no information between the contour lines without
the data were converted to grid format and interpolated using the
Once the data was interpolated, the data was reprojected
to UTM NAD 83. The interpolation map shows unique precipitation
at each grid point rather than the value half way between any two
lines which is shown on the contour map. No DEM was used to
the interpolation routine to maintain the integrity of the original
map to the degree possible. Only the contour lines were
There are some differences between the interpolated results and the
results which can be seen just west of the LWQD and on the west side of
the Bridger mountains. These differences serve as a caution that
the actual precipitation at a given grid point may be different than
interpolated value. No attribute table is presented
the attribute at each location is simply mean annual precipitation for
the period 1961-1990 expressed in inches.
Another widely accepted source of precipitation data is that
by PRISM. This information is reported on a map at the web site
in item two. This data reflects computer interpolation of
data. PRISM does not include NRCS Snow data and the maps show the
entire state so that generalization to a site is difficult.
other sites with raw climate data are hyper linked for the interested